An air conditioner implying a cooler, a heater or both of them is classified into a window type and a split type. In the case of the cooler, a split type air conditioner includes an indoor unit installed indoors for cooling a room, and an outdoor unit connected to the indoor unit through refrigerant pipe lines and installed outdoors to contact air, for performing condensation heat exchange on a refrigerant gas in a condenser by using external air as a cooling medium, and supplying the condensed refrigerants to an evaporator of the indoor unit through the refrigerant pipe lines. The indoor unit is composed of the evaporator for performing cooling heat exchange for evaporating the refrigerants and absorbing evaporation heat from internal air, and a ventilating fan for circulating internal air, and the outdoor unit is composed of a compressor for compressing the refrigerant gas and supplying the compressed gas to the condenser, the air-cooled condenser for condensing the refrigerant gas from the compressor, and a cooling fan for forcibly ventilating external air to the air-cooled condenser to cool and condense the refrigerant gas. The compressor, the air-cooled condenser and the cooling fan of the outdoor unit are installed in an outdoor unit casing composing the outer appearance. The conventional hexahedral outdoor unit casing has an air suction unit for sucking air to the air-cooled condenser at its three sides, and an air discharge unit for externally discharging air absorbing condensation heat from the refrigerant gas by the heat exchange in the air-cooled condenser on its top surface.
However, the conventional outdoor unit for the air conditioner is restricted in installation spaces due to high density and strict environment regulations of cities, and increases civil applications due to noise and heat. Especially, a common residential area such as large-scaled apartment buildings regulates the outdoor units to be installed in indoor verandas to improve the appearance and prevent noise.
In order to solve the foregoing problems, Japanese Laid-Open Patent Publication 6-101873 suggests an air conditioner mounted building where an  indoor unit of an air conditioner is installed indoors or adjacent to a room intended to be air-conditioned, and an outdoor unit of the air conditioner is installed outdoors, wherein an opening is formed on the outer wall or roof, a louver is installed in the opening, the outdoor unit of the air conditioner is positioned in the louver, and suction/discharge of the indoor unit is performed through a gap between louver plates.
In addition, Japanese Laid-Open Patent Publication 3-213928 discloses a wall built-in type outdoor unit for an air conditioner including an outdoor unit main body for the air conditioner which is built in the wall and which includes a frame having the same size and thickness as the wall, a suction hole for heat exchange air installed on the same surface as the outdoor unit main body, and a discharge hole for heat exchanged air.
However, the conventional arts relate merely to technologies for inserting the outdoor unit into a space formed on an outer wall of a building. That is, there has never been suggested a structure suitable for a common residential building using a plurality of built-in or front suction/discharge type outdoor units, or a structure for efficiently installing or repairing the outdoor units.
Moreover, when the outdoor unit increased in size due to large air conditioning capacity is formed in a front suction/discharge type, the outdoor unit has a smaller suction unit area than a conventional three-surface suction type outdoor unit, to increase a suction resistance and reduce heat exchange efficiency.